1. Field of the Invention
The present invention relates to a buried-waveguide-type light receiving element and manufacturing method thereof, more particularly, to a buried-waveguide-type light receiving element used in an optical communication system or the like, and manufacturing method thereof.
2. Description of the Related Art
With the rapid increase in demand for communication, the development of communication systems of higher capacities has been pursued, which requires designing optical communication devices so as to increase the processing speed, reduce the size, improve the efficiency and reduce the manufacturing cost.
Optical communication transmission systems use light in two wavelength bands as signal light: a 1.3 μm band having a center wavelength of 1.3 μm, and a 1.55 μm band having a center wavelength of 1.55 μm.
Signal light in the 1.55 μm band has a low optical fiber loss and is used as signal light in a long-distance communication system, which is, for example, called an intercity communication (trunk system) and used for communication between large cities such as Tokyo and Osaka.
On the other hand, signal light in the 1.3 μm band has a higher optical fiber loss but has reduced wavelength dispersion and is used as signal light in a short-distance communication system, which is called in-city communication and used for communication in a large city. Signal light in the 1.3 μm band is also used in a system for communication between a base station and each home called an access system.
Light receiving elements having such a structure that signal light enters the element through a cleaved end face have been developed and mass-produced as a semiconductor light receiving element used in a receiving module of an optical communication system. Conventional semiconductor light receiving elements of this kind include a loading-type semiconductor light receiving element (see, for example, Japanese Patent Laid-Open No. 2003-332613, section [0016] and FIGS. 1, 2, and 3). In this structure, light is caused to enter a guide layer transparent to incident light on the cleaved end face and is guided to a photoelectric conversion section (light absorption layer) formed at a distance of several microns from the incidence section, and light oozing out of the guide layer in the layer thickness direction (evanescent light) is photoelectrically converted in the photoelectric conversion section. This photoelectric conversion is in an indirect form such that the concentration of photocurrents in the vicinity of the incidence end surface is reduced. This arrangement therefore has the advantage of ensuring that a reduction in response speed or breakdown of the light receiving device cannot occur easily even when a high intensity of light is input.
On the other hand, since light oozing out of the guide layer in the layer thickness direction is photoelectrically converted, obtaining high sensitivity requires a substantial waveguide length in theory. However, if the wavelength length is increased for the purpose of obtaining high sensitivity, the element capacity of the light receiving element is increased and there is a possibility of failure to achieve the desired high-speed response performance. That is, the sensitivity and high-speed response are in a trade-off relationship.
Also, there is a need to cover side surfaces of the guide layer and the photoelectric conversion section with a film of a non-semiconductor material (e.g., SiN film) having a high refractive index ratio to the materials forming the guide layer and the photoelectric conversion section in order to improve optical confinement in the guide layer and to thereby reduce loss due to radiation to portions other than the photoelectric conversion section. However, a recombination level or the like can occur easily at the semiconductor/non-semiconductor interface. In some situation, there is an anxiety about degradation of the guide layer end surface on which light is concentrated and the photoelectric conversion section to which a high electric field is applied.
Known light receiving elements proposed to solve these problems include a buried-waveguide-type light receiving element having a construction in which a waveguide layer is embedded in a Fe-doped InP (hereinafter referred to simply as Fe—InP) (see, for example, “40 Gbps waveguide type PD for mounting flip chip” Eitaro Ishimura, Masaharu Nakamichi and others, The 49th applied physics consociated lecture 2002 (Heisei 14) Spring lecture proceedings (2002.3 Tokai University), p. 1152, 27a-ZG-7).
In this construction, an optical confinement layer and a light absorption layer are embedded in Fe—InP constituted of a semiconductor and protected to ensure high reliability.
Also the element has such a structure that light directly enter the light absorption layer through a window layer. Therefore high sensitivity can be obtained even if the wavelength length is not so long, and high-sensitivity and high-speed-response characteristics can be obtained.
Another known disclosed art is a waveguide-type semiconductor light receiving element in which a 0.6 μm thick boundary layer of an n-optical confinement layer adjacent to a light absorption layer and a 0.3 pn thick boundary layer of a p-optical confinement layer adjacent to the light absorption layer are each formed as a non-doped layer (see, for example, Japanese Patent Laid-Open No. 10-303449, section [0030] and FIGS. 1 and 2).
Still another known disclosed art is a loading-type semiconductor light receiving element in which an i-InAlGaAs guide layer (wavelength composition 1.3 μm, layer thickness 0.2 μm) is formed below a light absorption layer, which has an increased depletion layer and a reduced junction capacity, and which is capable of high-speed response (see, for example, Japanese Patent Laid-Open No. 2001-168371, section [0030] and FIGS. 1 and 2).
A further known disclosed art is a waveguide-type light receiving element in which a multilayer structure having a light absorption layer p-type doped, light guide layers provided on opposite sides of the light absorption layer, the light guide layer on one side being uniformly p-type doped, the light guide layer on the other side having a low-concentration layer (e.g., an undoped layer) and an n-doped layer successively formed from the light absorption layer, is provided in mesa form on a semi-insulating InP substrate, and in which light horizontally enters the layer structure (see, for example, Japanese Patent Laid-Open No. 11-112013, sections [0004], [0008], [0009] and [0012] and FIG. 1).
Still a further known disclosed art is a semiconductor light receiving element in which an n-InGaAsP light guide layer is provided on a semi-conducting InP substrate, and a waveguide structure including an n-InP electron travel layer in mesa form, an InGaAsP layer in an undoped and n-type two-layer structure and a p-InGaAs light absorption layer (see, for example, Japanese Patent Laid-Open No. 2000-124493, sections [0004], [3008], [0009] and [0012] and FIG. 1).
Still a further known disclosed art is a waveguide-type avalanche photodiode for 40 Gbps communication (see, for example, “Waveguide-type avalanche photodiode for 40 Gbps communication” Shogo Shimizu, Kazuhiro Shiba and others, Shingaku-Giho, IEICE Technical Report OCS2006-40, OPE2006-93, LQE2006-82(2006-10), pp. 11-15).
In the conventional buried-waveguide-type light receiving element having a waveguide embedded in a Fe—InP layer, however, there is a problem that there is a possibility of mutual diffusion of a p-type dopant and Fe being caused by regrowth of the Fe—InP layer after the formation of the waveguide to cause an increase in dark current between the light absorption layer and the n-type optical confinement layer, i.e., an increase in leak current. As a method for preventing this, increasing the layer thickness of the light absorption layer is conceivable. With this method, however, there is a problem that a reduction in high-speed response and deterioration in high optical input resistance are caused.